It is well known that automatic transmissions for vehicles comprise a plurality of clutches. As a transmission shifts through its various "gears" or "ranges," associated clutches are engaged and disengaged. In most transmissions, the clutches are independent of each other such that the prior engagement of one clutch does not impact the engagement of another. However, in some transmissions the engagement of one clutch lowers the capacity of another. For example, gear schemes of transmissions such as those shown in U.S. Pat. No. 4,070,927 are such that the operation of engaging one of the clutches is impacted by the prior engagement of another. Specifically, in such prior art patent the C1 clutch is nested in the C2 clutch. Accordingly, when the C1 clutch is engaged, the force available to apply the C2 clutch is reduced according to the formula: EQU C2 force=(C2 pressure)(C2 piston area)-(C1 pressure)(C1 piston area).
As a consequence, any transmission shift requiring application of the C2 clutch while the C1 piston cavity is pressurized is characterized by limited C2 clutch capacity.
In transmissions such as those taught by U.S. Pat. No. 4,070,927, the 3-4 range shift of the transmission is the weak link in the overall transmission torque capacity because clutch C2 is brought into engagement at a time that clutch C1 is already engaged.
There is a need in the art for an apparatus and technique which allows for improved clutch capacity control for nested clutch automatic transmission to assure sufficient capacity for the oncoming clutch to optimize the oncoming shift. Further, the art requires an apparatus and technique which provides such an improved clutch capacity control in a nested clutch automatic transmission which is adaptive, maintaining the length of the shift within a defined range.